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Saturday, July 6, 2013

July 6, 2013: Heat Wave To End Monday

*** Daily forecast updates will be resumed starting today. ***

Forecast Highlights:

The heat and humidity observed starting from July 4th continued today as a strong ridge persists over the western Atlantic Ocean. The ridge will stay in place on Sunday with similarly hot and humid conditions before flattening out on Monday as a frontal system approaches, with temperatures slightly cooling down but with humid conditions expected to continue as well as a risk of scattered storms through the rest of the week.

Over the last two days, a strong western Atlantic ridge developed and spread into the eastern US, with a southwesterly flow bringing a warm and humid air mass into the region. Although the ridge is expected to slightly weaken on Sunday while starting to shift southeast, the nearest shortwave will remain far enough away to keep most of the area dry except for possible isolated evening storms inland.

Temperatures are expected to be similar to those of today, peaking in the upper 80s to low 90s inland and the low to mid 90s for the rest of the area with a light SW wind. Along with continued humid conditions with dew points in the low 70s, heat index values are expected to reach the mid to upper 90s inland and the upper 90s to low 100s for the rest of the area.

Monday - Tuesday: Cooling Down, Some Storms

The upper level low that produced occasional rounds of showers and storms last week was originally stationary over the region, but was pushed back to the central US as the strong western Atlantic ridge developed and expanded west. By the early week, ridging aloft will flatten and shift east, allowing the upper level low that has been stuck in the Ohio Valley region for the last few days to make progress to the east, with a shortwave moving through the region on Monday night. This approaching shortwave, along with moderate instability and lapse rates, will trigger scattered storms across the region on Monday afternoon and evening, with the highest probability of storms north and west of NYC. With the instability, a slight risk of strong storms is expected, some of them severe, with heavy rainfall and gusty winds the main risk, although widespread cloud cover is expected to prevent more significant destabilization and a higher risk of severe weather, also taking into consideration a lack of significant bulk shear. With the ridge shifting southeast and an increase in cloud cover, temperatures will slightly cool down into the mid to upper 80s across the area, although humid conditions are expected to continue with the humid air mass still in place, with heat index values likely in the low 90s.

The shortwave will depart the region and weaken on Tuesday, with weak ridging briefly returning into the region along with a weak surface high pressure as the next shortwave approaches the Midwest. This will lower the storm risk on Tuesday, with partly sunny skies and highs slightly warmer than those of Monday, peaking in the mid to upper 80s, possibly low 90s in some locations, although isolated storms cannot be ruled out late in the day west of NYC especially with weak warm air advection. Humid conditions will continue, but with slightly less humidity compared to today.

Wednesday - Beyond: Cold Front, Widespread Storms Return

The next trough will approach the region from the northwest on Wednesday and Thursday, bringing the next storm system into the region. As the trough approaches the nearly stationary western Atlantic ridge, it will slow down and amplify, with the cold front at the surface slowing down as well. With the cold front nearby, widespread scattered storms are expected on Wednesday and Thursday, mostly from NYC and further north/west. Given the moderate instability and somewhat supportive wind shear and lapse rates, some of the storms could be severe, with the highest risk to the north/west of the area but possibly including parts of the area as well, especially on Thursday. Temperatures are expected to remain in the mid to upper 80s with continued humidity.

By the late week into next weekend, ridging in the western Atlantic is likely to expand back west towards the region, which would lead to another scenario of a frontal boundary sticking over the eastern half of the US. There is model uncertainty regarding exactly what happens, with the CMC failing to show the ridge expanding back west, instead showing the cold front quickly clearing the region, while the GFS stalls the front overhead with another round of frequent rain/storms and the ECM is more aggressive in pushing the frontal boundary west, bringing the Bermuda high pressure and another round of heat/humidity back into the region. Last week had a similar scenario where the medium range models underestimated the westward extent of the ridge, which pushed the frontal boundary into the Ohio Valley as opposed to keeping it stationary over the East; at this time I am siding with a GFS/ECM compromise idea of the front continuing to produce showers and storms in the region while making slow progress to the west, although this part of the outlook is still subject to change as it is in the longer range. Temperatures are likely to end up in the lower half of the 80s in the late week due to the cloud cover and rain, gradually warming up afterwards as ridging likely expands back west; should a scenario closer to the ECM verify with a stronger western Atlantic ridge returning, another round of heat and humidity may be possible. Stay tuned for more information on the longer range outlook over the next few days.

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